Vessels

Blood Vessels

• Overview of the cardiovascular system with focus on blood vessels.

Objectives

• Understand the blood vessels' functions and components.
  • Major functions and organization of the cardiovascular system.
  • Cellular and histological components of the cardiovascular system.
  • Homeostatic imbalances of the cardiovascular system.
• Discuss factors regulating heart rate, stroke volume, cardiac output, total peripheral resistance, and their effects on blood pressure.

Blood Vessel Types and Structure

• Blood vessels operate in a closed system, starting and ending at the heart.
• Three major types of blood vessels:
  • Arteries
  • Function: carry blood away from the heart.
  • In systemic circulation, arterial blood is oxygen-rich, while in pulmonary circulation, arterial blood is oxygen-poor.
  • Arteries branch into smaller vessels.
  • Veins
  • Function: carry blood toward the heart.
  • In systemic circulation, venous blood is oxygen-poor, while in pulmonary circulation, it is oxygen-rich.
  • Veins merge into larger vessels.
  • Capillaries
  • Function: microscopic vessels that allow exchange between blood and tissues.
  • Structure: walls are 1 or 2 cells thick, facilitating exchange of substances.

General Structure of Blood Vessels

• Blood vessels consist of three layers (tunics):
  • Tunica Interna (Tunica Intima):
  • Endothelial layer, possibly includes a subendothelial layer in larger vessels.
  • Tunica Media:
  • Contains smooth muscle and elastic fibers; allows significant changes in blood flow and pressure.
  • Tunica Externa (Tunica Adventitia):
  • Composed of collagen fibers for protection and reinforcement; contains vasa vasorum (blood vessels for larger vessels).

Comparison of Blood Vessel Types

• Arteries vs. Veins:
  • Arteries:
  • Thicker tunica media, narrower lumen compared to veins.
  • More elastic and resilient due to more elastic and collagen fibers.
  • Resistant to blood pressure changes.
  • Veins:
  • Thicker tunica externa, larger lumen than arteries.
  • Less elastic and collagen fibers, leading to collapse when no blood is present.
• Capillaries:
  • Only have tunica intima, allowing rapid gas and nutrient exchange.

Details of Arteries

• Elastic (Conducting) Arteries:
  • Largest arteries with large lumens; contain many elastic fibers to accommodate the stretch and recoil during the cardiac cycle.
• Muscular (Distributing) Arteries:
  • Medium-sized arteries directing blood flow to specific organs; capable of vasoconstriction and dilation.
• Arterioles:
  • Smallest arteries controlling systemic blood pressure and blood flow.

Clinical View: Atherosclerosis

• A progressive disease affecting elastic and muscular arteries.
  • Causes: endothelial injury due to infection, trauma, hypertension.
  • Inflammatory response leads to atheroma formation, thickening the tunica intima and narrowing the lumen.
  • Risk factors: hypercholesterolemia, smoking, hypertension.
  • Treatments: angioplasty, coronary bypass surgery.

Capillaries Overview

• Structure:
  • Average length of 1 mm, with a lumen about a quarter the size of a hair.
  • Erythrocytes travel single file due to their small diameter.
  • Facilitate optimal exchange of gases, nutrients, and hormones.
  • Types of capillaries:
  • Continuous Capillaries:
    • Endothelial cells form a continuous lining with intercellular clefts for small molecules.
    • Found in muscle, skin, lungs, CNS.
  • Fenestrated Capillaries:
    • Have small pores (fenestrations) allowing for filtration; found in the kidneys.
  • Sinusoids (Discontinuous Capillaries):
    • Contain large gaps allowing transport of large substances; found in bone marrow, liver, spleen.

Capillary Beds

• Function:
  • Composed of groups of capillaries functioning together, fed by metarterioles.
  • True capillaries branch from metarterioles, and the capillary bed connects to the postcapillary venule.
• Control Mechanism:
  • Precapillary sphincters regulate blood flow into capillaries; only a portion of capillary beds are open at any given time.
• Perfusion:
  • Amount of blood entering capillaries per unit time per gram of tissue, expressed in mL/min/g.

Venous System: Structure and Function

• Venules:
  • Formed when capillary beds unite; are leaky and facilitate the movement of fluids and white blood cells into the bloodstream.
  • Structure varies with size; larger venules have smooth muscle layers.
• Veins:
  • Categorized as small, medium-sized, and large veins.
  • Structure: similar to arteries but with larger lumen and thinner walls.
  • Contain valves to prevent pooling of blood and ensure unidirectional flow toward the heart.
• Volume Distribution:
  • At rest, 70% of blood volume resides in the systemic circulation.
  • 55% in systemic veins, 10% in systemic arteries, 5% in systemic capillaries.
  • Other distributions: 18% in pulmonary circulation, 12% in heart.

Factors Aiding Venous Return

• Venous blood pressure alone is inadequate, aided by:
  • Skeletal muscle pump: contractions increase venous return.
  • Respiratory pump: changes in thoracic pressure during breathing assist blood flow into thoracic veins.
  • Valves: prevent backflow during venous return.

Bulk Flow

• Bulk flow involves the movement of fluids down the pressure gradient.
  • Filtration: Fluid moves from blood into interstitial spaces; occurs at the arterial end of capillaries.
  • Reabsorption: Fluid moves from interstitial spaces back into the blood; occurs at the venous end of capillaries.

Net Filtration Pressure (NFP)

• NFP is calculated as: $NFP = (HP<em>b - HP</em>if) - (COP<em>b - COP</em>if)$
  • HP = hydrostatic pressure; COP = colloid osmotic pressure.

Lymphatic System

• The lymphatic system picks up excess fluid that is not reabsorbed at the venous end of capillaries, normally about 15% of fluid.
• It filters this fluid and returns it to the venous circulation.